In the above-mentioned ABS, an annular magnet (encoder) provided with N-poles and S-poles alternately in the circumferential direction is fixed to an inner ring, i.e. a driven wheel, of a hub bearing. A magnetic sensor is provided on a fixed side, thereby detecting magnetism change following rotation of the annular magnet and determining rotating condition of wheels. A cover is attached to the fixed side in order to prevent damage of the annular magnet caused by dirty water and dust, referring to the following Patent Literature 1 as an example. Such a cover is constituted with non-magnetic metal easily transmitting magnetism, for example, SUS304, aluminum, copper, brass, or the like, for accuracy of magnetic detection, because the cover is provided between the annular magnet and the magnetic sensor. The non-magnetic metal is pressed to form a predetermined shape like a cap of the cover.